Purine nucleoside phosphorylase (PNP) catalyses the phosphorolytic cleavage of ribo- and deoxyribonucleosides, for example, those of guanine and hypoxanthine to give the corresponding sugar-1-phosphate and guanine, hypoxanthine, or other purine bases.
Humans deficient in purine nucleoside phosphorylase (PNP) suffer a specific T-cell immunodeficiency due to an accumulation of dGTP and its toxicity to stimulated T lymphocytes. Because of this, inhibitors against PNP are immunosuppressive, and are active against T-cell malignancies. Clinical trials are now in progress using 9-(3-pyridylmethyl)-9-deazaguanine in topical form against psoriasis and in oral form for T-cell lymphoma and immunosuppression (BioCryst Pharmaceuticals, Inc). The compound has an IC.sub.50 of 35 nM for the enzyme. In animal studies, a 50 mg/kg oral dose is required for activity in a contact sensitivity ear swelling assay in mice. For human doses, this would mean approximately 3.5 grams for a 70 kg human. With this inhibitor, PNP is difficult to inhibit due to the relatively high activity of the enzyme in blood and mammalian tissues.
Nucleoside and deoxynucleoside hydrolases catalyse the hydrolysis of nucleosides and deoxynucleosides. These enzymes are not found in mammals but are required for nucleoside salvage in some protozoan parasites. Purine phosphoribosyltransferases (PPRT) catalyze the transfer of purine bases to 5-phospho-.alpha.-D-ribose-1-pyrophosphate to form purine nucleotide 5'-phosphates. Protozoan and other parasites contain PPRT which are involved in essential purine salvage pathways. Malignant tissues also contain PPRT. Some protozoan parasites contain purine nucleoside phosphorylases which also function in purine salvage pathways. Inhibitors of nucleoside hydrolases, purine nucleoside phosphorylases and PPRT can be expected to interfere with the metabolism of parasites and therefore be usefully employed against protozoan parasites. Inhibitors of PNP and PPRT can be expected to interfere with purine metabolism in malignant tissues and therefore be usefully employed against malignant tissues.
It is an object of the invention to provide pharmaceuticals which are very effective inhibitors of PNP, PPRT and/or nucleoside hydrolases.